PICROTOXIN ELIMINATES FREQUENCY-SELECTIVITY OF AN AUDITORY INTERNEURON IN A BUSH-CRICKET

AN1, an auditory interneuron in the bushcricket Ancistrura nigrovittat a, is narrowly tuned to the male song frequency (similar to 15 kHz). I t receives pronounced inhibitory input at frequencies below and, more prominently, above this fundamental frequency. It is also subject to s ide-dependent inhibition producing asymmetric response functions for l eft-and right-side stimulation. In addition, intensity-response functi ons of AN1 peak as stimulus intensities increase. Application of the G ABA(A) channel-blocker picrotoxin eliminates all subthreshold inhibito ry postsynaptic potentials, revealing underlying excitation that is pa rticularly obvious in the high-frequency range. Excitatory thresholds close to the song frequency remain unchanged by picrotoxin. Thus a spe cifically tuned neuron is shown to become broadly tuned after eliminat ion of frequency-dependent inhibition. Although average maximum respon se strength is increased by 150% after picrotoxin application, at male song frequencies a slight reduction of the responses is still present at high intensities. Side-dependent inhibition remains largely unaffe cted by picrotoxin, suggesting that side-and frequency-dependent inhib itions are caused by different transmitters from different neurons.